Organic light emitting diodes (OLEDs) are an emerging display technology. In essence an OLED (or organic electroluminescent device) comprises a thin organic layer or stack of organic layers sandwiched between two electrodes, such that when a voltage is applied, visible light is emitted. At least one of the electrodes must be transparent to visible light.
There are two principal techniques that can be used to deposit the organic layers in an OLED: thermal evaporation and solution processing. Solution processing has the potential to be the lower cost technique due to its potentially greater throughput and ability to handle large substrate sizes. However, several manufacturing issues still have to be resolved before solution processing of OLEDs can fulfil its potential. In a multi-colour or full-colour display the emissive organic layers need to be patterned according to the pixel layout. High-resolution displays require a high-resolution pattern for the emissive layer. To date, solution-processing techniques for patterning the emissive layer are far from ideal.
In many cases, the most efficient OLED devices have multi-layer structures (fluorescent emitter: e.g. U.S. Pat. No. 5,719,467 (Hewlett-Packard 1995), EP 0,921,578 (CDT 1998), U.S. Pat. No. 6,048,573 (Kodak 1998), U.S. Pat. No. 6,069,442 (Kodak 1997), U.S. Pat. No. 5,554,450 (Kodak 1995); phosphorescent emitter: e.g. WO 00/57676 and U.S. Pat. No. 6,303,238). Such multi-layer structures can be formed by thermal evaporation, but when solution-processing techniques are used, depositing a second layer may wash away the first layer.
It has been recognised that if a photolithographic technique could be successfully applied to the patterning of the organic layers in an OLED then this would offer many benefits. Photolithographic techniques are established in other industries and can give good resolution and high throughput. However the attempts to use photolithography during the formation of the organic layers in OLEDs have all had only very limited success.
BASF (U.S. Pat. No. 5,518,824) discusses the principle of forming an OLED using a crosslinkable charge-transporting material. The proposed functional groups are acrylates, vinylethers and epoxides. The material is deposited from solution, and then exposed to UV light, which crosslinks the material making it insoluble. Subsequent luminescent or electron transporting layers can be deposited on top of the insoluble layer. BASF mentions that if the UV exposure is carried out through a mask, then the exposed areas will be insoluble and the unexposed areas still soluble, and developing (washing) this film in solvent will remove the unexposed material, leaving the insoluble patterned material. However, this patterning is not demonstrated. BASF discuss doping the film with a fluorescent dye or using a crosslinkable fluorescent dye (U.S. Pat. No. 5,922,481) to form the light-emitting layer. The EL device results reported by BASF from its crosslinked devices are very poor. The two devices reported, which have crosslinked but un-patterned light emitting layers, give light emission only at 87 V and 91 V, respectively, both of which are entirely unacceptable operating voltages for an OLED. Canon (EP 1146574 A2) also demonstrate an OLED with a crosslinked emissive film, but there is still no demonstration of a patterned emissive layer. Further, Bacher et al. (Macromolecules 1999, 32, 4551-4557) demonstrated photo-crosslinking of a hole-transporting (acrylate derivative of triphenylene) material. They produced a patterned photo-crosslinked hole-transport layer on to which they deposited an emissive layer (tris(8-hydroxyquinoline)aluminium: Alq3), and made a functioning OLED device. However, they had not developed a technique for photo-lithographically patterning the emissive layer, and unless the emissive layer can also be patterned, only a monochrome device can be formed. The problem with acrylates in all the prior art is that although the acrylates can give very high resolution in the patterning process, they quench luminescence. The quenching of fluorescence by carbonyl groups is well known (Becker, Theory and Interpretation of Fluorescence and Phosphorescence, Wiley Interscience, NY 1969).
Other authors suggest crosslinking materials by thermally-initiated processes. While such processes do form insoluble films allowing subsequent layers to be deposited on top, they do not allow patterning of the layer. IBM has a patent (U.S. Pat. No. 6,107,452) on thermally/photochemically-induced crosslinking of polymers for use, for example, in light-emitting devices. No patterning was demonstrated.
Bayerl et al. (Macromolecules 1999, 20, 224-228) used crosslinked oxetane-bisfunctionalized N,N,N′,N′-tetraphenyl-benzidine as the hole-transporting material in a two-layer device. However, they did not pattern the hole-transporting material. Further work on oxetanes by Meerholz et al. (WO 02/10129) uses cationic photopolymerization to form crosslinked layers. In one instance the emissive layer was patterned. But in many cases the photoacid generated during the polymerization would attack other components of an OLED, in particular organometallic materials, and therefore such procedures would not generally be appropriate for the formation of patterned crosslinked emissive layers in OLED devices.
Photo-polymerisable thiol/ene systems are known for various applications such as printing plates and protective coatings. In these prior applications of thiol/ene systems the resulting polymers have been insulators. Most of the thiol/ene systems mentioned in the prior art contain non-conjugated carbonyl groups rather than aliphatic thiols, as aliphatic thiols can retain a nasty smell. In particular PETMP (Pentaerythritol tetrakis(3-mercaptopropionate) is commonly used as the thiol component (e.g. U.S. Pat. No. 5,100,929 and U.S. Pat. No. 5,167,882).
The present invention is directed to OLEDs that solve some of the problems in the prior art.